Membrane pumps of this kind are commercially available. They are used to deliver small volumes of a liquid very accurately while maintaining a separation between the liquid delivered and the moving parts of the pump. In membrane pumps an impermeable elastic membrane is held immovably at its edges while its central section is subjected to a stroke movement. For this purpose the central section of the membrane is connected to a core or piston formed as a plunger which is surrounded by an excitation coil. On excitation (or energisation) the piston is attracted into the excitation coil against the return force of the membrane or another means of producing spring tension. On de-energisation the spring-loading returns it to its starting position. During the stroke movement when the excitation coil is excited the medium to be delivered is sucked through an inlet opening into the swept space formed on the side of the membrane remote from the piston between the membrane and a cover plate securing it. Suitably there is a valve in the suction opening in the cover plate which opens only in the direction of suction. Most simply this is a spring-loaded flap. During the other stroke movement the volume sucked in is discharged again through another opening which includes a valve opening only in the direction of discharge, which in the simplest case is again a spring-loaded flap. This ensures that sucking-in can only occur during the one displacing movement and discharge only during the other displacing movement.
A disadvantage of commercially available metering pumps of this kind, however, is that the membranes used have to be relatively stiff so that they do not bend during the discharge stroke during which they have to deliver against pressure. As a result of this the force needed to displace the membranes must be large in order to overcome the high return force or spring-loading provided by such membranes. A further disadvantage is that the membranes age and become softer with time. Because of this the volume delivered per stroke changes, which is very undesirable. This is particularly undesirable if high-precision delivery is involved, for example of very small quantities in the medical field. A particular example is the delivery of dialysate to a dialyser in haemodialysis. In such an application it is essential to ensure that too much dialysate is not supplied to the dialyser under any circumstances as otherwise dialysate could get into the circulatory system of a patient, which is extremely dangerous. On the other hand the amount of dialysate supplied must not be too small, as the blood purification can then not be carried out completely. Finally, it is necessary, particularly in this type of application, to ensure that the dialyser is always and continuously supplied with an amount of fresh dialysate equal to the amount of used dialysate which is removed. For this purpose so-called balancing systems are used which are supplied continuously with fresh dialysate by means of ordinary feed pumps and the continuous flow is regulated by means of controlled valves and a bypass line.